This invention relates to a composite sheet useful as a disposable backing sheet of a thermosensitive recording adhesive label sheet and to a method of preparing the composite sheet. The present invention is also directed to a thermosensitive recording adhesive label sheet having the composite sheet as a disposable backing sheet thereof.
Recently, thermosensitive recording adhesive labels have been used in a wide variety of fields in the forms of, for example, labels, seals, stickers, tags and emblems. A thermosensitive recording adhesive label sheet is composed of a thermosensitive recording sheet peelably supported on a disposable backing sheet through an adhesive layer. The backing sheet generally includes a paper layer on which a release liner is provided. Wood-free paper, plain paper (e.g. glazed paper or kraft paper) or glassine paper is generally used as the paper layer.
The paper layer of such disposable backing sheets is desired to have improved disintegratability for recycle use. In this respect, glassine paper is ill-suited for recycle use because of its poor disintegratability. To improve disintegratability, proposals have been made in which the freeness of the glassine paper is adjusted. For example, Japanese Laid-Open Patent Publication No. H07-109699 proposes the use of a glassine paper having a Cobb size value (30 seconds) of 35-65 g/m2 and made of a wood pulp having a Canadian standard freeness value of 130 to 240 ml. Japanese Laid-Open Patent Publications No. H09-031898 proposes the use of a treated paper obtained by passing a glassine paper made of a wood pulp having a Canadian standard freeness value of 130 to 240 ml through a nip between a pair of rolls at 10-50 kg/cm, followed by application of an aqueous solution of polyvinyl alcohol. Japanese Laid-Open Patent Publications No. H09-217299 discloses a backing sheet having a glassine paper made of a mixture of 90 to 60 parts by weight of a highly disintegrated pulp having a Canadian standard freeness value of 100 to 250 ml with 10 to 40 parts by weight of a lowly disintegrated pulp having a Canadian standard freeness value of at least 400 ml.
The above backing sheets, which generally show better disintegratability as compared with glassine paper, have a problem because the light transmittance thereof is not good. Namely, label sheets using the above backing sheets are ill-suited for use with printers provided with a light transmission-type label position sensor. To improve the light transmittance, therefore, it is necessary to reduce the thickness of the backing sheet. This results in difficulty in cutting thermosensitive recording medium on the backing sheet into labels and in difficulty in removing cut labels therefrom. While a calendar treatment may improve light transmittance, it is necessary to use a special calendar installation in order to attain a desired level of light transmittance.
The above backing sheets have an additional problem with respect to releasing properties. Namely, when a release liner is formed on the paper layer, pin holes are apt to form in the liner, so that an adhesive coating applied to the release liner penetrates into the backing sheet so that the releasability of labels becomes poor.
To cope with the above problem, Japanese Laid-Open Patent Publication No. H08-144198 proposes the use of paper layer having formed thereon a barrier layer including at least one resin selected from polyacrylamide-acrylate copolymers, styrene-butadiene copolymers and copolymers of polyvinyl alcohol with a polystyrene-acrylate polymer. Japanese Laid-Open Patent Publication No. H10-204795 proposes providing, over a glassine paper, an undercoat layer including a pigment having a particle diameter of 0.4 xcexcm or less and a water soluble polymer. Japanese Laid-Open Patent Publications No. H11-1897 discloses a backing sheet including a machine glazed paper layer, a coating of a water-soluble polymer provided on the paper layer, and a release liner provided on the polymer coating. The polymer coating has a Cobb size value (60 seconds) of 10 to 40 g/m2 and a Beck""s smoothness of 300 to 1000 seconds. Although the release properties of the above backing sheets are improved, the known backing sheets are not fully satisfactory with respect to light transmittance and disintegratability.
It is, therefore, the prime object of the present invention to provide a composite sheet which is useful as a backing sheet of a thermosensitive recording adhesive label sheet, which is devoid of the drawbacks of the conventional composite sheets and which exhibits both good light transmittance and good disintegratability.
Another object of the present invention is to provide a method of preparing the above composite sheet.
It is a further object of the present invention to provide a thermosensitive recording adhesive label sheet having the above composite sheet as a disposable backing sheet thereof.
In accomplishing the foregoing objects, there is provided in accordance with the present invention a composite sheet comprising a paper layer made of a wood pulp having a Canadian standard freeness value of 350 to 500 ml and a transmittance of at least 7% for a light with a wavelength of 940 to 960 nm, a transparency improving layer provided over one side of said paper layer so that said paper layer overlaid with said transparency improving layer exhibits a transmittance of at least 11% for a light with a wavelength of 940 to 960 nm, and a release liner provided over an opposite surface of said transparency improving layer from said paper layer. In another aspect, the present invention provides a method of preparing the above composite sheet, comprising the steps of:
applying a coating of a transparency improving layer-forming liquid onto a paper layer made of a wood pulp having a Canadian standard freeness value of 350 to 500 ml and a transmittance of at least 7% for a light with a wavelength of 940 to 960 nm to form a transparency improving layer of said paper layer,
bringing a heat roller into rolling contact with said transparency improving layer to dry said transparency improving layer so that said paper layer overlaid with said transparency improving layer exhibits a transmittance of at least 11% for a light with a wavelength of 940 to 960 nm, and
forming a release liner on an opposite surface of said transparency improving layer from said paper layer.
The present invention further provides a thermosensitive recording adhesive label sheet, comprising the above composite sheet, an adhesive layer provided on said release liner, and a thermosensitive recording sheet provided on said adhesive layer such that said thermosensitive recording sheet with said adhesive layer is peelable from said composite sheet.